One of the frequently used conventional channel estimation methods is a direct sequence spread spectrum method. In order to improve the accuracy of channel estimation in this method, the width of chips used for spreading should be reduced (i.e. wide bandwidth), while the amount of spreading should be increased. In addition, the frequency reference used in this method should have a very small error so that the precision of channel estimation, particularly estimation of time delay values can be increased. An example of this method can be found in the case of a GPS (global positioning system).
In order to improve the accuracy of time delay measurement among multi-path channel parameters, the frequency bandwidth of used signals should be increased. This is because an error in time delay measurement is inversely proportional to a used frequency bandwidth. Particularly, in order to reduce the measurement error to lower than a threshold value in a method for increasing the frequency bandwidth of signals, the frequency bandwidth should be increased up to a predetermined value. However, there is a difficulty in meeting this requirement due to various frequency regulations.
In addition, a high degree of precision is required for the frequency reference used in the sender and receiver, in order to improve the accuracy of time delay measurement among multi-path channel parameters. To this end, a high-precision oscillator should be used. However, this increases the overall cost of the system.
In summary, various frequency regulations deters the increase of the frequency bandwidth for the purpose of precise measurement, and the burden of rising cost makes it impractical to manufacture high-performance sender and receiver having a wideband chip rate.